Off-axis reflective optical systems, both focal and afocal, have found use especially in applications where a broad spectral coverage and high throughput is required. Several examples of off-axis reflective optics are found in the prior art, including U.S. Pat. No. 6,274,868. The off-axis nature of the mirror elements eliminates the central obscurations found in conventional on-axis design forms such as the standard Cassegrain telescope. The advantages of the off-axis optical system forms are offset by the fact that the mirror alignments must be extremely precise, as there are no rotational degrees of freedom to take advantage of during manufacture. The required positional precision unfortunately also precludes the prior art techniques of providing manual or automatic mechanical adjustments of the positions of the mirrors in order to achieve effects such as focusing, zooming, etc. In the case of an off-axis reflective afocal optical system, it is generally not economically feasible to provide a mechanical adjustment of the existing mirror system which alters the magnification power without reducing the optical image quality. One prior art solution is described in U.S. Pat. No. 5,477,395 wherein two pre-aligned afocal assemblies are nested along a common rotational axis, such that one or other assembly can be rotated into place over the pupil in order to select a magnification. A disadvantage of U.S. Pat. No. 5,477,395 however, is that the described axis of mechanical rotation for the FOV switch, either horizontally or vertically, does not coincide with the optical axis through the pupil of the system. Because of that limitation, the mechanical switching system cannot serve a dual purpose to adjust image roll.
Image roll is a phenomenon common in aircraft sensor systems, where the aircraft pitch, yaw, and roll relative to the earth can cause the sensor projection on the ground to rotate. In many cases, the pilots prefer to have a “de-roll” capability to offset the variable effects of aircraft attitude. For navigation use, de-roll can be used to ensure the earth's horizon as seen by the sensor remains in same position as seen out the window. In digital mapping applications, de-roll can be used to ensure each ground sample image is aligned symmetrically with the next sequential sample image so that they can be stitched together to form a composite map. In the prior art for nested multiple field of view afocals, a de-roll capability would require a second rotational mechanism.